This invention relates to a magnetic recording medium. More particularly, the present invention is concerned with a high-density magnetic recording medium which is excellent in recording and reproducing properties.
In recent years, an increasing demand for a magnetic recording medium, such as a magnetic disk, having a higher density brought about a tendency towards reduction in the thickness of a coating film containing a ferromagnetic substance. Specifically, the thickness of the coating film which was 4 to 5 .mu.m in the past has been reduced to 1 .mu.m or less in recent years (see the textbook of the 615th institute of the Japan Society of Mechanical Engineers, p. 84, January 1986).
Such a magnetic disk can be produced by a method which comprises dispersing ferromagnetic powder in a binder such as a composition comprised of an epoxy resin, a phenolic resin and a vinyl resin or cellulose to prepare a paint, applying the paint on a substrate to form a coating film, and curing the coating film (see Japanese Patent Laid-Open Nos. 66803/1979, 183625/1982, and 136028/1985). The paint used in the above-mentioned method may contain a filler such as alumina.
In the above-mentioned prior art method, to consideration was paid on a problem that the reduction in the thickness of the coating film caused a decrease in the amount of magnetic powder per unit area and, therefore, brought about a decrease in the reproduced output.
As mentioned above, a magnetic disk is produced using a paint containing magnetic powder dispersed therein. It is noted in this connection that, when the proportion of the magnetic powder is high, the magnetic powder coagulates so that it cannot be uniformly dispersed. As can be seen also from the above-mentioned Japanese Patent Laid-Open Specifications, the content of magnetic powder in the coating film in the case where a uniform dispersion of the magnetic powder was attained was 55 to 60% by weight with only one exception of uniform dispersion being attained when the content exceeded 60% by weight, i.e., when the content was 65% by weight. When the content of the magnetic powder was 65% by weight, not only the coating film is weak but also various cares should be taken on the storage of the paint.
In the case of a magnetic tape, the magnetic powder may be used in an amount of as much as about 80% by weight. This is because the thickness of the magnetic tape is as large as 4 to 5 .mu.m and is produced using a paint having a high viscosity, i.e., a paint having a high binder concentration, which makes it possible to uniformly disperse a larger amount of the magnetic powder.
On the other hand, in a high-density magnetic disk, the coating film should be thin, which requires the use of a paint having a low viscosity in the formation thereof. Therefore, in this case, the coating film is formed using a paint having a low binder concentration, i.e., a paint of which the magnetic powder content cannot be remarkably increased. As mentioned above, since the conventional high density magnetic disk has a coating film having a low magnetic powder content, it was difficult to improve the recording and reproducing properties.